Since the announcement of the first commercial installation of Octafining in Japan in June, 1958, this process has been widely installed for the supply of p-xylene. See "Advances in Petroleum Chemistry and Refining" volume 4 page 433 (Interscience Publishers, New York 1061). That demand for p-xylene has increased at remarkable rates, particularly because of the demand for terephthalic acid to be used in the manufacture of polyesters.
Typically, p-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Density Freezing Boiling Lbs./U.S. Point.degree. F. Point.degree. F. Gal. ______________________________________ Ethyl benzene -139.0 277.0 7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 282.4 7.23 O-xylene -13.3 292.0 7.37 ______________________________________
Principal sources are catalytically reformed nahthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range 10 to 32 wt. % ethyl benzene with the balance, xylenes, being divided approximately 50 wt. % meta, and 25 wt. % each of para and ortho.
In turn, calculated thermodynamic equilibra for the C.sub.8 aromatic isomers at Octafining conditions are:
______________________________________ Temperature 850.degree. F. ______________________________________ Wt. % Ethyl benzene 8.5 Wt. % para xylene 22.0 Wt. % meta xylene 48.0 Wt. % ortho xylene 21.5 TOTAL 100.0 ______________________________________
An increase in temperature of 50.degree. F. will increase the equilibrium concentration of ethyl benzene by about 1 wt. %, ortho-xylene is not changed and para and meta xylenes are both decreased by about 0.5 wt. %.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethyl benzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para xylene is separated from the mixed isomers by fractional crystallization.
As commercial use of para and ortho xylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes.
Octafining process operates in conjunction with the product xylene or xylenes separation processes. A virgin C.sub.8 aromatics mixture is fed to such a processing combination in which the residual isomers emerging from the product separation steps are then charged to the isomerizer unit and the effluent isomerizate C.sub.8 aromatics are recycled to the product separation steps. The composition of isomerizer feed is then a function of the virgin C.sub.8 aromatic feed, the product separation unit performance, and the isomerizer performance.
The isomerizer unit itself is most simply described as a single reactor catalytic reformer. As in reforming, the catalyst contains a small amount of platinum and the reaction is carried out in a hydrogen atmosphere.
Octafiner unit designs recommended by licensors of Octafining usually lie within these specification ranges:
______________________________________ Process Conditions Reactor Pressure 175 to 225 PSIG Reactor Inlet Temperature Range 830-900.degree. F. Heat of Reaction Nil Liquid Hourly Space Velocity 0.6 to 1.6 Vol/Vol/Hr. Number of Reactors, Downflow 1 Catalyst Bed Depth, Feet 11 to 15 Catalyst Density, Lb/Cu. Ft. 38 Recycle Circulation, Mols Hydrogen/ Mol Hydrocarbon Feed 7.0 to 14.0 Maximum Catalyst Pressure Drop, PSI 20 ______________________________________
It will be apparent that under recommended design conditions, a considerable volume of hydrogen is introduced with the C.sub.8 aromatics. In order to increase throughput, there is great incentive to reduce hydrogen circulation with consequent increase in aging rate of the catalyst. Aging of catalyst occurs through deposition of carbonaceous materials on the catalyst with need to regenerate by burning off the coke when the activity of the catalyst has decreased to an undesirable level. Typically the recommended design operation will be started up at about 850.degree. F. with reaction temperature being increased as needed to maintain desired level of isomerization until reaction temperature reaches about 900.degree. F. At that point the isomerizer is taken off stream and regenerated by burning of the coke deposit.
Actual operation of Octafining varies from the recommended ideal in some cases. In the case of one commercial Octafiner, temperature has been reduced for increased throughput such that a cycle is begun at 760.degree. F. and ended at 860.degree. F. Concurrently, hydrogen recycle is reduced to 6.5 mols of H.sub.2 per mol of hydrocarbon charge. Cycle time between regenerations is cut to 3 months at these conditions.
During regeneration, burning proceeds very slowly with diluted oxidizer medium in order to minimize damage to the catalyst. The several days required for regeneration are non-productive and the catalyst after regeneration is at a reduced activity level. For example, an operation at a hydrogen to hydrocarbon recycle ratio of 6.5 results in a cycle life of about 3 months between regenerations with replacement of the catalyst required after about 1 year, four cycles.
In a typical plant for utilization of Octafining, a mixture of C.sub.8 aromatics is introduced to an ethyl benzene tower wherein the stream is stripped of a portion of its ethyl benzene content, to an extent consistent with retaining all the xylenes in the feed stream without unduly expensive "superfractionation." Ethyl benzene is taken overhead while a bottom stream, consisting principally of xylenes, together with a significant amount of ethyl benzene, passes to a xylene splitter column. The bottoms from the xylene splitter constituted by o-xylene and C.sub.9 aromatics passes to the o-xylene tower from which o-xylene is taken overhead and heavy ends are removed. The overhead from the xylene splitter column is transferred to conventional crystallization separation. The crystallizer operates in the manner described in Machell et al., U.S. Pat. No. 3,662,013 dated May 9, 1972.
Because it's melting point is much higher than that of the other C.sub.8 aromatics, p-xylene is readily separated in the crystallizer after refrigeration of the stream and a xylene mixture lean in p-xylene is transferred to an isomerization unit. The isomerization charge passes through a heater, is admixed with hydrogen and the mixture is introduced to the isomerizer.
Isomerized product from the isomerizer is cooled and passed to a high pressure separator from which separated hydrogen can be recycled in the process. The liquid product of the isomerization passes to a stripper from which light ends are passed overhead. The remaining liquid product constituted by C.sub.8 + hydrocarbons is recycled in the system to the inlet of the xylene splitter.
It will be seen that the system is adapted to produce maximum quantities of p-xylene from a mixed C.sub.8 aromatic feed containing all of the xylene isomers plus ethyl benzene. The key to efficient operation for that purpose is in the isomerizer which takes crystallizer effluent lean in p-xylene and converts the other xylene isomers in part to p-xylene for further recovery at the crystallizer.
Among the xylene isomerization processes available in the art, Octafining has been unique in its ability to convert ethyl benzene. Other xylene isomerization processes have required extremely expensive fractionation to separate that component of C.sub.8 aromatic fractions. As will be seen from the table of properties above, the boiling point of ethyl benzene is very close to those of p- and m-xylene. Complete removal of ethyl benzene from the charge is impractical. The usual expedient for coping with the problem is an ethyl benzene separation column in the isomerizer-separator loop when using catalyst other than those characteristic of Octafining. It will be seen that Octafining does not need this expensive auxiliary to prevent build up of ethyl benzene in the loop. This advantageous feature is possible because the Octafining catalyst converts ethyl benzene.
The Octafining process has been extensively discussed in the literature, for example:
1. Pitts, P. M., Connor, J. E., Leun, L. N., Ind. Eng. Chem., 47, 770 (1955).
2. Fowle, M. J., Bent, R. D., Milner, B. E., presented at the Fourth World Petroleum Congress, Rome, Italy, June 1955.
3. Ciapetta, F. G., U.S. Pat. No. 2,550,531 (1951).
4. Ciapetta, F. G., and Buck, W. H., U.S. Pat. No. 2,589,189.
5. Octafining Process, Process Issue, Petroleum Refinery, 1st Vol. 38 (1959), No. 11, Nov., p. 278.
A typical charge to the isomerizing reactor (effluent of the crystallizer) may contain 17 wt. % ethyl benzene, 65 wt. % m-xylene, 11 wt. % p-xylene and 7 wt. % o-xylene. The thermodynamic equilibrium varies slightly with temperature. The objective in the isomerization reactor is to bring the charge as near to theoretical equilibrium concentrations as may be feasible consistent with reaction times which do not give extensive cracking and disproportionation.
Ethyl benzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethyl benzene to benzene and diethyl benzene, hydrocracking of ethyl benzene to ethane and benzene and hydrocracking of the alkyl cyclohexanes.
The rate of ethyl benzene approach to equilibrium concentration in a C.sub.8 aromatic mixture is related to effective contact time. Hydrogen partial pressure has a very significant effect on ethyl benzene approach to equilibrium. Temperature change within the range of Octafining conditions (830.degree. to 900.degree. F.) has put a very small effect on ethyl benzene approach to equilibrium.
Concurrent loss of ethyl benzene to other molecular weight products relates to % approach to equilibrium. Products formed from ethyl benzene include C.sub.6 + naphthenes, benzene from cracking, benzene and C.sub.10 aromatics from disproportionation, and total loss to other than C.sub.8 molecular weight. C.sub.5 and lighter hydrocarbon by-products are also formed.
The three xylenes isomerize much more selectively than does ethyl benzene, but they do exhibit different rates of isomerization and hence, with different feed composition situations the rates of approach to equilibrium vary considerably.
Loss of xylenes to other molecular weight products varies with contact time. By-products include naphthenes, benzene, toluene, C.sub.9 + aromatics and C.sub.5 and lighter hydrocracking products.
Ethyl benzene has been found responsible for a relatively rapid decline in catalyst activity and this effect is proportional to its concentration in a C.sub.8 aromatic feed mixture. It has been possible then to relate catalyst stability (or loss in activity) to feed composition (ethyl benzene content and hydrogen recycle ratio) so that for any C.sub.8 aromatic feed, desired xylene products can be made with a selected suitably long catalyst use cycle.
Another xylene isomerization which has achieved widespread commercial use is low pressure operation in vapor phase. Temperatures employed are in the same range as for Octafining, in the neighborhood of 850.degree. F. Pressures are only that required to equal pressure drop through the downstream recovery towers, heat exchanges and the like. For all practical purposes, this is an atmospheric pressure reaction with reactor inlet pressure of about 30 pounds per square inch, gauge. The catalyst is essentially silica-alumina, the acid amorpohous heterogeneous catalyst employed in a number of such acid catalyzed processes. Several advantages for that type of isomerization will be immediately apparent.
The unit cost of catalyst is drastically reduced by ommission of platinum. At these low pressures, the reactor vessels are made of inexpensive steel and need no structural provision for resisting pressure stress. The process is practical without introduction of molecular hydrogen and needs no auxiliaries for manufacture and recycle of that gas. These features greatly reduce capital and operating costs and have made the low pressure process essentially competitive with Octafining despite the requirement for large vessels at low pressure and low space velocity and the operating disadvantages inherent in the process.
A primary drawback of low pressure vapor phase isomerization as practiced heretofore is its low tolerance for ethyl benzene in the charge. The catalyst will convert ethyl benzene only at high severities such that unacceptable loss of xylene occurs by disproportionation.
Low pressure isomerization as practiced heretofore accepts a further disadvantage in that the catalyst rapidly declines in activity due to deposition of "coke," a carbonaceous layer masking the active sites of the porous silica-alumina catalyst presently conventional in this operation. The coke can be removed by burning with air to regenerate the activity of the catalyst. Continuity of operation is achieved by the well-known "swing reactor" technique employing two or more reactors, one of which is on stream while burning regeneration is conducted on a reactor containing spent catalyst which has lost activity by coke deposition. Cycles of two to four days are common practice using one reactor on stream for that period and then shifting to a freshly regenerated vessel.
Present commercial practice involves many large plants of both the Octafining and low pressure types in a loop of p-xylene separation and recycle of other isomers, together with such quantity of ethyl benzene as may be present, through isomerization and back to p-xylene recovery. The commercial options presently in use are Octafining at high pressure with large quantities of hydrogen or low pressure (essentially atmospheric) isomerization with complicated cycling of a swing reactor and necessity for expensive distillation to remove ethyl benzene from the charge to some acceptable level, usually about 5%.
A further alternative heretofore described is isomerization in liquid phase at a pressure adequate to maintain that phase. Highly active zeolite catalysts are effective under these conditions and demonstrate long cycle life, possibly because precursors of coke are dissolved by the reactant liquid and flushed from the reactor before deterioration to coke. See, for example, Wise, U.S. Pat. No. 3,377,400; Bowes et al., U.S. Pat. No. 3,578,723; and Haag et al., U.S. Pat. No. 3,856,871.
It is further known that zeolite ZSM-5 is a very effective catalyst for isomerization of xylenes. See Argauer et al., U.S. Pat. No. 3,790,471; Burress, U.S. Pat. No. 3,856,873; Morrison, U.S. Pat. No. 3,856,872; and Haag et al., supra. It should be noted that Burress describes a wide range of operating conditions and demonstrates effectiveness of the catalyst at (1) low temperature, high pressure and (2) high temperature, low pressure operation over zeolite ZSM-5. On this state of the art, zeolite ZSM-5 can be expected to function effectively in low pressure, vapor phase isomerization, and indeed it does. That zeolite and the related zeolites are defined hereinafter by silica/alumina ratio, constraint index and crystal density. Further, in the absence of hydrogen, these zeolites accumulate coke on stream in the manner to be expected from knowledge in the art to require short cycle times, when operating outside the bounds of limits now found essential to prolonged on-stream periods.